In order to support higher data rate and spectrum efficiency, the Third Generation Partnership Project (3GPP) introduced improvements to long term evolution (LTE).
In LTE (i.e., LTE release 8/9), Single Carrier Frequency Division Multiple Access (SC-FDMA) transmission was selected for uplink (UL) direction. The specific implementation is based on Discrete Fourier Transform Spread Orthogonal Frequency Division Multiplexing (DFT-S-OFDM). For the purpose of this application, either term may be used interchangeably. A wireless transmit/receive unit (WTRU) in the UL will transmit only on a limited, contiguous set of assigned sub-carriers in an FDMA arrangement. Only for illustration purposes, if the overall OFDM signal or system bandwidth in the UL is composed of useful sub-carriers numbered 1 to 100, a first given WTRU would be assigned to transmit its own signal on sub-carriers 1-12, a second given WTRU would transmit on sub-carriers 13-24, and so on. An evolved NodeB (eNodeB or eNB) would receive the composite UL signal across the entire transmission bandwidth from one or more WTRUs at the same time, but each WTRU would only transmit into a subset of the available transmission bandwidth. DFT-S OFDM in the LTE UL was selected by 3GPP Radio Layer 1 (RAN1) as a form of OFDM transmission with the additional constraint that the time-frequency resource assigned to a WTRU must consist of a set of frequency-consecutive sub-carriers. In the LTE UL, there is no DC sub-carrier (unlike the downlink (DL)). Frequency hopping may be applied in one mode of operation to UL transmissions by a WTRU.
WTRUs transmit their UL data (and in some cases their control information) on the physical uplink shared channel (PUSCH). The transmission of the PUSCH is scheduled and controlled by the eNodeB using the so-called uplink scheduling grant, which is carried on physical downlink control channel (PDCCH) format 0. As part of the uplink scheduling grant, the WTRU receives control information on the modulation and coding set (MCS), transmit power control (TPC) command, uplink resources allocation (i.e., the indices of allocated resource blocks), etc. Then, the WTRU will transmit its PUSCH on allocated uplink resources with corresponding MCS at transmit power controlled by the TPC command.
Similar to LTE DL, reference signals for channel estimation are also needed for the LTE UL to enable coherent demodulation of PUSCH (or PUCCH) at the eNodeB. These reference signals are referred to as UL demodulation reference signals (DRS). They are always transmitted together with and covering the same frequency band as PUSCH (or PUCCH).
In order to allow for the eNodeB to estimate UL channel quality for UL scheduling, sounding reference signals (SRS) may be transmitted in UL, not associated with transmission of PUSCH and PUCCH. In the frequency domain, SRS transmissions may cover the frequency band that is of interest for the frequency domain scheduling. When an SRS is to be transmitted in a subframe, it occupies the last SC-FDMA symbol of the subframe. If a WTRU is transmitting SRS in a certain subframe, then the last symbol of the subframe is then not used for PUSCH transmission by any WTRU within the cell.
In order for the eNodeB to perform reliable channel estimation for frequency-scheduling for each UL, the transmit power for SRS (and other channels) is controlled. LTE methods may not account for SRS transmission where UL Multiple Input Multiple Output (MIMO) and carrier aggregations techniques are utilized. UL MIMO and carrier aggregation may also effect the determination of SRS parameters and power settings.